The phenomenon of superconductivity, that is, zero electrical resistance, possessed by many metals at near absolute zero temperatures, has received steadily increasing attention in recent years due to the development of materials which exhibit this property at sufficiently high temperatures while carrying relatively high currents in the presence of sufficiently great magnetic fields as to be of commercial utility. See generally, "Superconductors in Electric Power Technology", Geballe et al, Scientific American, Vol. 243 No. 5 pp. 138-172 (November, 1980). Among the more useful of the superconducting materials developed to data are the intermetallic compounds Nb.sub.3 Sn and V.sub.3 Ga. These materials have sufficiently good superconductive properties as to render them attractive in the development of useful electrical machinery. However, the manufacture of these intermetallic compounds is complicated both by the nature of the compounds themselves, which are metastable phases not readily manufacturable by simple chemical processes, and which are so brittle that the bending of a conductor formed of either of these compounds is substantially precluded, and by the preferred design of conductors using these materials, which generally require many individual filaments of the superconductive material to be imbedded in a matrix of a non-superconductive material, preferably a metal having high electrical conductivity such as pure Cu.
Early attempts at manufacture of filamentary wires of, for example, Nb.sub.3 Sn are shown in Berghout et al U.S. Pat. No. 3,496,622 in which mixed Nb and Sn powders are packed in a wrapped sheath of, for example, Mo; this structure is then drawn to a fine wire size and heat treated to form the Nb.sub.3 Sn compound desired. However, this method does not lend itself readily to the production of multifilamentary wires; moreover, the Mo sheath is undesirable as it is preferable to have a material of high electrical conductivity such as Cu in direct contact with or disposed in close juxtaposition to the Nb.sub.3 Sn superconductive material. An improvement on the Berghout et al method was shown in U.S. Pat. No. 3,541,680 to Verrijp in which a small quantity of Cu, Ag, Au, Pt or Pd is added to the Nb and Sn powder mixture to inhibit the formation of intermetallic compounds other than the Nb.sub.3 Sn such as Nb.sub.6 Sn.sub.5 or NbSn.sub.2.
More recently developed processes for the manufacture of Nb.sub.3 Sn have generally involved the so-called bronze process in which rods or wires of Nb are dispersed throughout a matrix consisting of a CuSn bronze. The assembly is worked to a desired final size and heat treated at which time Nb.sub.3 Sn is formed at the interfaces between the Nb rods and the bronze matrix by diffusion of the Sn from the bronze. See, for example, U.S. Pat. No. 3,918,998, assigned to the assignee of the present application. Refinements to the bronze process include providing a quantity of good electrical conductor such as pure Cu in close proximity to the Nb.sub.3 Sn filaments and isolating this pure Cu from diffusion of Sn which would destroy the high electrical conductivity of pure Cu by interposing a layer of material impermeable to Sn therebetween such as, for example, Ta; see, e.g., U.S. Pat. No. 4,205,199, also assigned to the assignee of the present invention. The same process is used to form multifilamentary V.sub.3 Ga; V rods are disposed in a CuGa bronze matrix.
The bronze process, while feasible, is not the ideal method of manufacture of a mutifilamentary superconductor of the A-15 type. Its chief drawback is the fact that the bronze used to supply Sn to the Nb rods or filaments for the formation of Nb.sub.3 Sn work hardens very quickly during the multiple metal working operations such as rolling, drawing, extrusion, swaging, and the like carried out to form the multifilamentary conductor, thus necessitating time consuming and costly annealing operations. Moreover, the amount of Sn which can be alloyed with Cu to form the bronze is limited to approximately 15%, as beyond this level, the bronze is prohibitively hard to work. Accordingly, a need remains in the art for an improved process for the manufacture of multifilamentary superconductors of the A-15 type.
During the development of the bronze process as outlined above, attention has continued to be paid to various powder metallurgical methods of manufacture of A-15 type superconductors. A tape of Nb.sub.3 Sn was manufactured by Lawrence Berkeley Laboratories by compacting Nb powder to form a tape having approximately 25% porosity. This tape was then immersed in molten Sn, which infiltrated the pores. Cold working was then carried out to form the tape to a final size, at which time a high temperature heat treatment was performed to form the Nb.sub.3 Sn product. A similar process could be used to form a rod-shaped conductor; thereafter, pluralities of such conductors could be jacketed in, e.g., Cu, packed together and extruded to form a multifilamentary Nb.sub.3 Sn superconductor. However, while this process avoids work hardening of bronze, it is unduly complex and expensive for reasonable quantity production of Nb.sub.3 Sn. A similar method, shown in U.S. Pat. No. 4,223,434 to Wang et al, yields Nb.sub.3 (Al,Ge) material (also an A-15 compound) and also involves manufacture of a porous Nb preform. The AlGe alloy is infiltrated into these pores; upon performance of a complex heat treatment, differing compounds of Nb with Ge and Al are formed in sequence. Again, this method is too elaborate to permit adaptation to economic volume production of Nb.sub.3 Sn.
Another alternative to the bronze process, shown in U.S. Pat. No. 4,224,735 to Young et al assigned to the assignee of the present invention, involves disposition of elemental Sn in layers not thicker than a predetermined dimension separated by Cu layers around a Cu matrix containing Nb filaments. This method allows working of a conductor precursor without work hardening of bronze, but is somewhat complex to implement on a commercial scale. Moroever, the Sn is not disposed uniformly with respect to the Nb.
A final possibility which has been suggested in the art but which has not achieved commercial success is shown in U.S. Pat. Nos. 3,838,503 and 3,829,963, among others. Here Sn is added to the exterior of a previously formed wire comprising multifilamentary Nb in a Cu matrix and diffused inwardly under a high temperature heat treatment. The Sn may be applied by dipping, plating or other means. This method is limited severely in the amount of Sn which can be caused to diffuse through such a matrix. In particular, the fact that Sn is not disposed uniformly throughout the conductor prohibits maximization of the Nb.sub.3 Sn formed, and hence limits the maximum current density of the eventual conductor.
Therefore, there exists a distinct need in the art for an improved method of formation of Nb.sub.3 Sn multifilamentary superconductors which does not involve working of a CuSn bronze, does not involve diffusion of Sn inwardly from the exterior of a Nb-in-Cu composite wire, and which does not involve the infiltration of a Nb matrix with Sn or other material which forms a superconductive compound with Nb.